Clemens, in U.S. Pat. Nos. 3,842,194, 3,842,217 and 3,909,517, has described a conductive video disc comprising a molded plastic disc having video and audio information in the form of geometric variations in a spiral groove in the disc surface. These discs are coated first with a conductive material, such as a metal, which acts as a first electrode and then with a dielectric layer, such as an inert polymer layer. A metal tipped stylus acts as a second electrode of a capacitor and the information signals are monitored by the stylus which notes changes in capacitance between the stylus and the disc surface as the information, in the form of depressions, passes beneath the stylus when relative motion is established between the disc and the stylus.
Further developments in this system have produced a video disc which is made of a conductive plastic material, e.g., a PVC copolymer resin containing sufficient amounts of conductive particles, such as carbon particles, so that the disc can provide capacitance readout. The carbon particles are surrounded by organic material such as the plastic resin, lubricants, etc. which provide a dielectric surface layer on the conductive particles. This development has eliminated the need for separate coatings of metal and dielectric on the plastic disc and thus has greatly improved the economics of the system. In other developments, video discs which do not require a grooved surface are also being improved. The stylus is maintained in synchronization with an information pattern by means of electrical signals on either side of the information track, rather than physically by means of the groove walls.
The carbon loaded PVC molding compositions known to date are unsatisfactory in certain respects, however The addition of large quantities of conductive carbon particles to a molding composition greatly increases the melt viscosity of the molten molding mixture and increases the brittleness of the molded disc. In order to obtain uniform disc replicas having very small and precise information patterns, large quantities of plasticizers and processing aids have been added to the molding composition. This unfortunately also contributes to a low heat distortion temperature of the resin and makes it dimensionally unstable in hot weather, leading to rippling and shrinkage which renders the disc unplayable. Thus, research has been continuing to improve the processability of the carbon loaded molding composition which increasing the heat distortion temperature and mechanical properties of the molded disc.